1. Field of the Invention
The present invention relates to a display device. Further, the present invention relates to an electronic device including the display device in a display portion.
2. Description of the Related Art
In recent years, as display devices such as a liquid crystal display device and an EL (electroluminescence) display device, an active matrix driving display device which includes thin film transistors (TFTs) in each pixel for higher definition is used in many cases. In an active matrix driving display device, a potential of a pixel electrode in each pixel can be independently controlled and thus, there is no crosstalk such as leakage of electric charge to an adjacent pixel, as in the case of a passive matrix driving display device in which pixels are controlled per line. Accordingly, a display device with little unevenness of display image and a higher contrast ratio can be manufactured.
As an example of a conventional active matrix display device, a structure and an operation of a pixel portion in a liquid crystal display device are described with reference to FIG. 24. FIG. 24 is a circuit diagram illustrating a structure of a conventional display device.
As illustrated in FIG. 24, the conventional display device includes a scanning line 700, a signal line 701, and a pixel. The pixel includes a switching transistor 702, a storage capacitor 703, and a capacitor of liquid crystal 704. The switching transistor 702 has a gate terminal, a source terminal, and a drain terminal. The gate terminal of the switching transistor 702 is electrically connected to the scanning line 700, and one of the source terminal and the drain terminal of the switching transistor 702 is electrically connected to the signal line 701. A first electrode of the storage capacitor 703 and a first electrode of the capacitor of liquid crystal 704 are electrically connected to the other of the source terminal and the drain terminal of the switching transistor 702.
Next, the operation of the conventional display device is described. At the time of writing a video signal, a signal is inputted from the scanning line 700 to the gate terminal of the switching transistor 702. When voltage applied between the gate and the source of the switching transistor 702 is equal to or higher than the threshold voltage of the switching transistor 702, the switching transistor 702 is an on state, and the video signal is inputted from the signal line 701 to the first electrode of the storage capacitor 703 and the first electrode of the capacitor of liquid crystal 704 through the switching transistor 702. In each of the storage capacitor 703 and the capacitor of liquid crystal 704 having the first electrode to which the video signal has been inputted, a potential of a second electrode is set in response to a signal from the outside, and thus a potential difference (voltage) between the potential of the first electrode and the potential of the second electrode is applied to the storage capacitor 703 and the capacitor of liquid crystal 704. In the capacitor of liquid crystal 704, liquid crystal molecules are controlled in accordance with the applied voltage, and display is performed.
An example of a method for driving the above display device includes frame inversion driving. In frame inversion driving, a signal whose polarity is inverted from a polarity of a signal inputted in one frame period is inputted to a capacitor of liquid crystal in the next frame period in order to prevent burn-in of the liquid crystal. For frame inversion, voltage twice as high as the writing voltage is applied between the source and the drain of the switching transistor 702. Thus, a high electric field is generated at the drain edge of the switching transistor 702, and carriers (hot carriers) accelerated by the high electric field degrades the transistor, resulting in increase in off-current and change in threshold voltage of the transistor.
When the off-current is increased due to degradation of the switching transistor, electric charge leaks from the storage capacitor 703 and the capacitor of liquid crystal 704 even if the switching transistor 702 is an off state (is in a period to hold a potential applied to the storage capacitor 703 and the capacitor of liquid crystal 704), and voltage applied to the liquid crystal becomes lower than a desired value. Accordingly, unevenness of display image occurs.
As an example of a technique for suppressing reduction in voltage of either of the capacitor of liquid crystal or the storage capacitor which is due to change in switching characteristics of the switching transistor in each pixel in the above liquid crystal display device, there is a structure in which a voltage control circuit electrically connected to the common electrode side of either of the capacitor of liquid crystal or the storage capacitor is provided (see Patent Document 1: Japanese Published Patent Application No. H5-216442).
In Patent Document 1, electric charge is compensated through the common electrode of a storage capacitor so as to adjust a potential of another electrode of the storage capacitor in accordance with change in switching characteristics of a switching transistor in each pixel, so that voltage applied to the storage capacitor is maintained at a predetermined value.
In an active matrix driving display device such as the above liquid crystal display device, since voltage held in a capacitor is changed from a predetermined value due to change in switching characteristics of a switching element or the like and thus, display image varies among pixels, a variety of circuits for suppressing change in voltage have been suggested.